For more than three decades, the number of transistors on a chip has grown exponentially, doubling on the average of every 18 months. With each new technology generation, the role of lithography has increased in importance not only because of the requirements for smaller feature sizes and tighter overlay, but also because of the increasing costs of lithography tools. Optical projection lithography and its extensions, e.g., water immersion, are expected to remain the lithographic technologies of choice until at least 2010. Extreme ultraviolet lithography (EUVL) extends optical lithography to a higher resolution and provides a larger depth of focus because it utilizes a shorter imaging wavelength (13.5 nm versus 193â248 nm) and employs a smaller numerical aperture (NA) imaging system (0.25â0.45 NA versus 0.93â1.35 NA). This chapter recounts the early years of EUVL development, from the first imaging with normal incidence multilayer (ML)-coated mirrors in 1981 to the beginning of EUVL commercialization efforts at the end of 1996.
Early concepts for EUVL emerged from research in Japan and the U.S. during the 1980s using soft x-rays in the 4-nm to 40-nm wavelength range. The results of the first demonstration of soft x-ray reduction lithography using multilayer-coated Schwarzschild optics were made public in 1986. The first demonstration of the technology's potential and of nearly-diffraction-limited imaging took place in 1990. Because of EUV lithography's potential to be utilized at ever-smaller feature sizes, a consortium of U.S. national laboratories, integrated device manufacturers, and private commercial companies combined to form the EUV Limited Liability Company (EUV LLC) in 1997 to guide and fund its commercialization. (For details, see Chapter 2 of this book.) Today, commercial EUV exposure-tool development is underway at ASML, Canon, and Nikon, and EUVL infrastructure development is under active development worldwide at a number of universities, national laboratories, and semiconductor consortia.
Most of the basic concepts needed for EUVL were demonstrated, and most of the EUV-specific critical issues were identified during the early years. Sufficient progress was made on all of the critical issues during this time period that none thereafter were regarded as a âshow stopperâ; several critical issues were resolved entirely. Two early success stories were in the fabrication and metrology of aspheric surfaces and in the application and metrology of reflective ML coatings. In the beginning, aspheric surfaces were not much more than laboratory curiosities, but eventually they could be fabricated with a controlled spectrum of surface heights spanning spatial wavelengths from the clear aperture of the part down to ~1 nm. Reflective ML coatings initially provided only marginally larger reflectances than a grazing incidence mirror, but later could provide normal-incidence reflectances close to the theoretical maximum. The work done on ML coatings for EUVL has spawned important research on the processes by which a single atomic layer can be formed and on the quality of the interface between materials. While some EUV-specific critical issues still have not been completely resolved, the questions that remain tend to involve cost and reliability rather than technical issues.
The major historical developments that took place from 1981 to 1992 are summarized in Sec. 1.2 and from 1993 to 1996 in Sec. 1.3. Other relevant developments that took place in Japan and Europe are summarized in Sec. 1.4. The development of the most important components and subsystems are described in Sec. 1.5, including the selection of the exposure wavelength, the design of reflective imaging systems, the fabrication and evaluation of aspherical mirrors, and the development of ML coatings and reflection masks, EUV resists, and EUV light sources. The most important EUVL conferences that took place during the early years, together with a list of the conference proceedings and technical journals that contain the majority of papers on EUVL from its beginning in the mid-1980s through 1996, are described in Sec. 1.6 and in Tables 1.1 and 1.2. A short summary of the current status of EUVL is presented in Sec. 1.7, followed by an extensive list of references.